Solid-state organic light-emitting diode (OLED) image display devices are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the device age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials may age at different rates, causing differential color aging and a display whose white point varies as the display is used. If some light-emitting elements in the display are used more than other, spatially differentiated aging may result, causing portions of the display to be dimmer than other portions when driven with a similar signal.
Referring to FIG. 2, a graph illustrating the typical light output of an OLED display device as current is passed through the OLEDs is shown. The three curves represent typical performance of the different light emitters emitting differently colored light (e.g. red, green and blue light emitters, respectively) as represented by luminance output over time or cumulative current. As can be seen by the curves, the decay in luminance between the differently colored light emitters can be different. The differences can be due to different aging characteristics of materials used in the differently colored light emitters, or due to different usages of the differently colored light emitters. Hence, in conventional use, with no aging correction, the display will become less bright and the color, in particular the white point, of the display will shift.
The rate at which light-emitting elements in OLED displays age is related to the amount of current that passes through the device and, hence, the amount of light that has been emitted from the display. U.S. Pat. No. 6,414,661 B1 issued Jul. 2, 2002 to Shen et al. describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light-emitting diodes (OLEDs) in an OLED display device, by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. This technique requires the measurement and accumulation of drive current applied to each pixel, requiring a stored memory that must be continuously updated as the display is used, requiring complex and extensive circuitry.
U.S. Pat. No. 6,504,565 B1 issued Jan. 7, 2003 to Narita et al., describes a light-emitting device which includes a light-emitting element array formed by arranging a plurality of light-emitting elements, a driving unit for driving the light-emitting element array to emit light from each of the light-emitting elements, a memory unit for storing the number of light emissions for each light-emitting element of the light-emitting element array, and a control unit for controlling the driving unit based on the information stored in the memory unit so that the amount of light emitted from each light-emitting element is held constant. An exposure device employing the light-emitting device, and an image forming apparatus employing the exposure device are also disclosed. This design also requires pixel usage accumulation and the use of a calculation unit responsive to usage information for each pixel, greatly increasing the complexity of the circuit design.
JP 2002278514 A by Numeo Koji, published Sep. 27, 2002, describes a method in which a prescribed voltage is applied to organic EL elements by a current-measuring circuit and the current flows are measured; and a temperature measurement circuit estimates the temperature of the organic EL elements. A comparison is made with the voltage value applied to the elements, the flow of current values and the estimated temperature, the changes due to aging of similarly constituted elements determined beforehand, the changes due to aging in the current-luminance characteristics and the temperature at the time of the characteristics measurements for estimating the current-luminance characteristics of the elements. Then, the total sum of the amount of currents being supplied to the elements in the interval during which display data are displayed, is changed so as to obtain the luminance that is to be originally displayed, based on the estimated values of the current-luminance characteristics, the values of the current flowing in the elements, and the display data. This design presumes a predictable relative use of pixels and does not accommodate differences in actual usage of groups of pixels or of individual pixels. Hence, correction for color or spatial groups is likely to be inaccurate over time.
US2004/0150590 entitled “OLED Display with Aging Compensation” by Cok et al describes an OLED display that includes a plurality of light-emitting elements divided into two or more groups, the light-emitting elements having an output that changes with time or use; a current measuring device for sensing the total current used by the display to produce a current signal; and a controller for simultaneously activating all of the light-emitting elements in a group and responsive to the current signal for calculating a correction signal for the light-emitting elements in the group and applying the correction signal to input image signals to produce corrected input image signals that compensate for the changes in the output of the light-emitting elements of the group. While it is suggested that each group may consist of an individual light-emitting element, the current measurement of individual light-emitting elements is time-consuming and may be difficult and inaccurate because the current through each element is typically very small. Alternatively, OLED systems that employ independent measurements of distinct groups of light-emitting elements over the entire OLED device are limited in their ability to deal with differential usage or light-emitter performance of individual elements within each group and cannot effectively compensate for such differential aging. Accordingly, it would be desirable to provide an aging compensation system wherein the speed and accuracy with which the current usage of individual light emitting elements may be measured is improved.